A power converter implemented by semiconductor switching devices, e.g. MOSFET (metal-oxide-semiconductor field effect transistor) or IGBT (insulated-gate bipolar transistor), is commonly used for converting an alternating current or a direct current and connecting different communication electrical source with electric-operated electrical source.
FIG. 1 shows a circuit of a conventional full-bridge alternating/direct current converter therein. The converter includes a rectifying circuit composed of six rectifier diodes D1 to D6, a filtering circuit composed of a filtering capacitor C11 for converting an input AC voltage into a smoothing DC voltage, a DC-to-DC circuit composed of four full-bridge semiconductor switching devices Q11 to Q14, a transformer T1, a set of rectifier diodes D7 to D10 and a LC filtering circuit composed of a capacitor C13 and a inductor L11. The AC/DC current converter shown in FIG. 1 often needs a large capacitor C11 as a filtering segment, the input current will not be in real state so as to reduce the converting efficiency when the capacitor C11 is disposed in the converter. Thus, the conventional converter having a large capacitor therein do not meet the requirement in different countries concerning the harmonic wave of the input current in an electric appliance. In order to meet the requirement of the harmonic wave of the input current, the converter needs an additional filter to be disposed in the filtering segment. This kind of converter is always built in a huge bulk to filter the harmonic wave because the input current has a lot of harmonic wave component therein, which is costly. However, the circuit is simple and the switching device in the converter is cheap to be used in manufacturing line. Thus, the circuit structure in such of converter is widely.
FIG. 2 shows a conventional boosting circuit structure in a rectifying circuit according to the prior art. A major switch Q21 is added to the rectifying circuit to be connected in parallel with six rectifier diodes D1 to D6, and further connected in series with a boosting diode D27 via a filtering capacitor C21. The working principle of the boosting circuit is described as the following descriptions.
When the major switch Q21 is turned-on, a three-phase voltage will be shortened by three input inductors La, Lb and Lc. A three-phase input current is proportional to each phase of the three-phase voltage when the boosting circuit is working. On the contrary, when the major switch Q21 is off, the output voltage will reduce the three-phase input current. If the turned-off time of the major switch is short enough to be ignored, the average value of respective input current of the three-phases input current to be inputted into the circuit is proportional to the respective voltage of the three-phases voltage. In addition, if the on/off frequency in the major switch is high enough to filter the harmonic wave, only a small filter could filter off the high harmonic wave in the boosting circuit. As a result, the AC to DC converter shown in FIG. 2 has a power factor correcting function therein. However, this converter still produces a higher output voltage. For example, when an input voltage 380 volts is inputted, a high voltage 1000 volts is outputted by the converter to meet the requirement of the IEC61000-3-2 safety standard.
A harmonic-wave inputting method is proposed to control the input harmonic wave so as to reduce the output voltage and meet the requirement of the safety standard of input harmonic wave. The wave head in the three-phase rectifying circuit can be used to control the duty ratio in the major switch. Theoretical and practical experiment proved that the harmonic wave in the input current and the output voltage outputted by the three-phase rectifying circuit could be greatly reduced by the harmonic-wave inputting method. In other word, the value of the output voltage can be reduced and also meet the requirement of the standard of input harmonic wave. For example, the output voltage can be reduced to 750 volts to meet the requirement of the IEC61000-3-2 safety standard by the harmonic-wave inputting method.
Although the harmonic-wave inputting method can reduce the output voltage from a converted circuit, the voltage is still high voltage, e.g. 750 volts, which is hard for general users to use such high voltage situation. Furthermore, the reduced voltage can be further converted by another DC/DC converter so as to reduce the high output convert into a useful circuit for the general users to operate. FIG. 3 shows a coupled circuit formed by the boosting circuit connected in series with a full-bridge DC/DC circuit. As can be seen in FIG. 3, the electrical power of the coupled circuit needs to be transmitted by two converting stages, and has the disadvantage that the soft turned-on switch cannot naturally operated. Thus, the electrical power cannot be efficiently converted in the coupled circuit.
It is attempted by the applicant to provide an electrical converter to overcome the problems as described above for reducing the harmonic wave and increasing the converting efficiency in the converter.